Semiconductor contact alloy



April 18, 1967' SEMI CONDUCTOR CONTACT ALLOY Filed Aug. 5, 1964 Wave M. Abercrombie Ernest C. Wurst, Jr.

- INVENTOR.

W. M ABERCROMBIE ETAL 3,314,830

United States Patent ce 3,314,830

Patented Apr. 18, 1967 wafer constitutes the collector, P-type layer 11 con 3,314,330 st-itutes the base, and alloy 14 forms the emitter. SEMICONDUCTOR CONTACT ALLOY With the exception of the emitter alloy, the above- Wava M. Abercrombie and Ernest C. Wurst, Jr., Dallas, described gallium arsenide transistor is representative of Tex., assignors to Texas Instruments Incorporated, 5 known compound semiconductor transistors. Hence the Dallas, -i 3 corporation of Delaware conventional processes for making such devices which in- Filed Aug. 3, 1964, Ser. No. 386,862

1 d th e ss st of etchin cleaninand diffus- 2 Claims. Cl. 148-33) C u e e 11 Ce ary ps g g ing have been omitted as they form no part of this invention.

This invention relates to contact materials for semi- 1O I h ti a1N P N gallium rsenide transistor, COndlJCtOI devices, SllCh as transistors, and more particuthe emitter is formed alloying a ontact conlarly to alloys used for the formation of the emitter of v r i N type impurities t th P-type layer 11. A conah Groups III-V Compound transistor, and more ventional N- type impurity used for this purpose is tin, specific-ally a gal m arsenide tIaHSiStOF- which acts as an electron donor in gallium arsenide and one y of p y g the q y figure of merit of 15 therefore may be used as the emitter contact. Howa transistor is by stating the fraction of the emitter ever, transistors produced using this conventional N-type rent which becomes collector current. This fraction is .i it t i t tly exhibit low value of beta (3) which called the current multiplication factor, alpha (a) When are generally attributed to poor emitter efiiciency a transistor 18 connected with the signal 111 the base circuit since galhurn arsen de is a compound of elements from in a common emitter arrangement, the current gain, beta 20 G o III a d V f the Pe iodi Table, it would ordi- (B), is the lailo 0f the Collector current to the base narily be expected that other elements from Group III rent and is Commonly eXPTessed y the general formula would enter the gallium arsenide crystal lattice substitu- (1:5 tionally, that is, by replacing the gallium. Further, since SI-nee alpha is 11 measure of emitter current which their outermost electron orbital, it would also be ex- Fmsses the collector junction it will seen h alpha pected that the substitution of other Group III elements is a product of at least three factors: (1) emltter for gallium in the gallium arsenide lattice would produce ciency (sometimes called injection efiiciency), (2) base I no substantial change in the conductivity or conductivitytransport efficiency, and (3) collector efficiency. It will type of the parent gallium arsenide compound. be understood that each of these factors is expressed in Yet in direct opposition to all expectations and terms of efliciency, hence can never have a value greater than unity. Consequently, alpha, being a product of these three factors, can be no higher than the lowest of these three efiiciencies.

Briefly, the present invention relates to a novel transistor emitter alloy containing indium and tin which, when alloyed to a thin P-type layer on an N-type wafer, particularly of N-type gallium arsenide, unexpectedly produces a gallium arsenide transistor with an advantageously high emitter eificiency.

high values of beta. Applicants know of no explanation for this phenomenon. While We do not wish to be bound indium advantageously improves the emitter efiiciency.

To test the efficiency of the tin-indium emitter contacts 40 for gallium arsenide signal translating devices, two groups single crystal N-type gallium arsenide was first polished It is raise an l e 0f the invention to Provide an and diffused with magnesium in a sealed ampoule for one N-P-N gallium arsenide transistor having a high emitter hour at 1100 C This diff i process produced a type diffused layer approximately 0.24 mil thick. The It is e filiiher l of this Invention to Provide a sheet resistance at the surface of the diffused layer measlium arsenide transistor with a high current multiplication d 51 53 h Th diffu ed layer was removed ifrom all except one surface of the slice, and the slice It is Y a further ohleci of this invention to Provide a was then cut into small dice or wafers approximately 20 gallium arsenide transistor having a novel emitter contact i b 20 il Th w fers were Separated i two and improved emitter efficiency. groups f equal b ThCSE and other objects, features, and advantages will Transisfers were made from one group by placing the N-type surface of the Wafers each on a thin strip of gold, containing one percent selenium by weight, which in turn 55 rested on a thin strip of platinum A two-mil diameter section of a gallium arsenide transistor device embodying sphere f 99 999% pure i was l d on h P- the P p e of the Present iIlVehh-Ohsurface. The assembly was then briefly (about 3 seconds) The transistor illustrated in the sole figure of the drawh d to approximately 450 c, on a v ti l strip ihg Comprises a Wafer 10 0f YP single y al galli m heater. During this heating step, the gold-selenium alloy arsenide having a yp layer 11 formed on its pp alloyed to both the N-type side of the gallium arsenide surface, for example by diffusion. A suitable ohmic conwafer and to h l i strip to form an h i p tact for example an alloy of gold and Zinc, is alloyed lector contact 13, 15. The tin sphere alloyed to the P- To the P layer 11 to form the base Contact All type layer to form an N-type emitter regrowth region 16. electrical lead 12a forms part of the contact 12. An emit- A goldinc (96% A 4% Z i 12 was th ft alloy 14, Containing till (which is an N- yp p n alloyed to the P-type surfiace to form an ohmic base conand indium in proportions as hereinafter described, is nection 12 and lead 120. A two-mil diameter silver wire also alloyed to the top surface of P-type layer 11 to form 17 was attached to the tin emitter sphere 14.

a barrier or rectifying junction therewith. A suitable Transistors were made from the second group of waters ohmic contact 13, 15, for example platinum combined in precisely the same manner as described above with the with a gold-selenium alloy, is attached and electrically single exception that two-mil diameter spheres of an alloy connected to the opposite surface of the wafer 10. It will of tin containing 0.05 percent indium by weight were subbe understood that in the transistor shown and described, stituted for the pure tin emitter spheres. The transistors made from both groups were then tested. The transistors from the first group (having pure tin emitters) exhibited beta values of less than unity, while those made from the second group and having the tin-indium emitter exhibited beta values ranging from five to 50 and values of F (cutoff frequency) in excess of 200 imegacycles.

As another example illustrating this invention, we have found that these advantageous results were also obtained with an emitter alloy which contained tin and 2.0 percent indium by weight. Thus it has been shown that this invention provides an emitter alloy which advantageously improves the electrical characteristics of N-PN gallium arsenide transistors.

It is to be understood that this invention is not limited to emitter alloys containing only tin and indium, but is expressly intended to include other emitter alloys in which tin is the N-type impurity. For example, the invention may be used to improve the electrical characteristics of gallium arsenide transistors using high temperature emitter contacts such as those described in U.S. Patent No. 3,012,175 to M. E. Jones et al. and assigned to the as signee of the instant application, which patent describes emitter contact alloys comprising substantially equal atomic proportions of tin and gold or substantially equal atomic proportions of tin and silver and tin-gold. In these alloys, tin is the N-type impurity. By the addition of about 0.05% to about 2.0% by weight of indium in accordance with the present invention, the high emitter etliciency of the present invention is combined with the high temperature advantages of the Jones et al. alloys to produce high temperature NP-N gallium arsenide transistors with improved current multiplication factors.

It is to be understood that the form of this invention,

ferred example of the same and that various changes may be resorted to without departing from the spirit and scope, of the invention as defined by the appended claims.

What is claimed is:

1. In an NPN gallium arsenide transistor comprising a body of gallium arsenide defining contiguous N- and P- regions and having a rectifying contact alloyed to said P-region forming an N-type conductivity region therein, the improvement wherein said rectifying contact is an alloy containing about 0.05% to about 2.0% by weight of indium, the remainder of said alloy consisting essentially of tin.

2. In an NP-N gallium arsenide transistor comprising a body of gallium arsenide defining contiguous N- and P- regions, and having a rectifying contact alloyed to said P-region forming an N-type conductivity region therein, the improvement wherein said rectifying contact is an alloy containing about 0.05% to about 2.0% by weight of indium the remainder of said alloy comprising substantially equal atomic proportions of tin and gold or substantially equal atomic proportions of tin and silver.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Hansen: Constitution of Binary Alloys, McGraw-Hill Book Co., New York, 1958, pages 860-863 relied on.

herewith shown and described, is to be taken as a pre- 35 DAVID RECK Primary Emmi/72"" CHARLES N. LOVELL, Examiner. 

1. IN AN N-P-N GALLIUM ARESINIDE TRANSISTOR COMPRISING A BODY OF GALLIUM ARESINIDE DEFINING CONTIGUOUS N-AND PREGIONS AND HAVING A RECTIFYING CONTACT ALLOYED TO SAID P-REGION FORMING AN N-TYPE CONDUCTIVITY REGION THEREIN, THE IMPROVEMENT WHEREIN SAID RECTIFYING CONTACT IS AN ALLOY CONTAINING ABOUT 0.05% TO ABOUT 2.0% BY WEIGHT OF INDIUM, THE REMAINDER OF SAID ALLOY CONSISTING ESSENIALLY OF TIN. 